1. Field of the Invention
The present invention relates to a resistance welding method and the structure of a resistance welding part obtained by the resistance welding method. Further, it pertains to a method for manufacturing an electronic component including the resistance welding method and the electronic component obtained by the method for manufacturing same.
2. Description of the Related Art
One may find the examples in which a process for joining a first metallic member comprising iron or an alloy containing iron (generically referred as an iron-based metal hereinafter) to a second metallic member comprising copper or an alloy containing copper (generically referred as a copper-based metal hereinafter) is applied in manufacturing an electronic component described, for example, in Japanese Examined Utility Model Publication No. 5-28751 or Japanese Unexamined Utility Model Publication No. 7-3128.
In more detail, the electronic component is provided with cap-shaped terminals put on its both ends and a central conductor placed on its center axis, and each cap-shaped terminal is electrically connected to the central conductor by applying resistance welding while the inner face of each cap-shaped terminal butts against each end face of the central conductor. The cap-shaped terminal serves as the first metallic member described above composed of the iron-based metal, and the central conductor serves as the second metallic member composed of the copper-based metal.
The iron-based metal and the copper-based metal have been selected as the materials of the cap-shaped terminal and the central conductor, respectively, by the following reasons.
In the electronic component having a specified structure as described in the foregoing utility model publications, the central conductor is electrically connected with an element by keeping elastic contact with an electrode formed on an inner face of a perforation hole of the element while the central conductor is received into the perforation hole provided in the element to be disposed around the central conductor. In addition, the element is mechanically positioned by the elastic contact. Accordingly, the central conductor is manufactured by rounding a metal plate comprising a copper-based metal such as phosphorus bronze and beryllium copper having a good electric conductivity and high elasticity into a cylinder.
The material of the cap-shaped terminal is required, on the other hand, to have a good resistance welding property with the central conductor described above as well as a relatively high mechanical strength. Therefore, an iron based metal such as an iron-nickel alloy is used for the material to have sufficient conductivity, oxidation resistance and corrosion resistance.
FIGS. 4A and 4B illustrate a method for joining the first-metallic member 1 comprising an iron-based metal to the second metallic member 2 comprising a copper-based metal by resistance welding.
As shown in FIG. 4A, the first metallic member 1 comprising an iron-based metal and the second metallic member 2 comprising a copper-based metal are at first prepared before applying resistance welding. A tin or silver film 3 is formed by plating on the surface of the first metallic member 1 in order to protect the iron-based metal comprising the first metallic member 1 from oxidation or corrosion, and in order to have a good solderability on the surface of the first metallic member 1.
Then, as shown in FIG. 4B, the first metallic member 1 is allowed to butt against the second metallic member 2, and an electric current is allowed to flow between the first metallic member 1 and the second metallic member 2 to form an alloy by fusing a part each of the first metallic member 1 and the second metallic member 2 by a heat generated by contact resistance between the first metallic member 1 and the second metallic member 2, thereby the first metallic member 1 is joined to the second metallic member 2.
After joining the two kind of the metallic members by resistance welding, an alloy layer 4 is formed along the interface between the first metallic member 1 and the second metallic member 2.
The tin or silver film 3 melts, or undergoes heat expansion or shrinkage, by the effect of the heat generated by welding. Accordingly, the tin or silver film 3 may be removed from the welded portion or cracks may be caused in the vicinity of the welded portion to expose the first metallic member in the vicinity of the welded portion.
However, since the alloy layer 4 formed by resistance welding as described above comprises an iron-copper alloy, it is readily corroded in an environment comprising corrosive substances such as water, halogens and acids. Consequently, corrosion of the alloy layer 4 progresses with time depending on the environment where the electronic component is used, when, for example, resistance welding is used for joining between the terminal member and the connecting conductor in the electronic component, sometimes resulting in shortening the service life of the electronic component.
When the surface of the first metallic member 1 is exposed from the tin or silver film 3 in the vicinity of the welded portion as hitherto described, it happens that iron contained in the first metallic member 1 corroded.
For preventing corrosion of the alloy layer 4, or the welded portion, from occurring, it may be contemplated to use the same material for the first metallic member 1 and the second metallic member 2. However, it is not too much to say that constructing the first metallic member 1 and the second metallic member 2 with the same material one another has no meaning so far as the iron-based metal is used for the first metallic member 1 and the copper-based metal is used for the second metallic member 2 as a measure for taking advantage of the characteristics of respective metals. Additionally, these metallic members 1 and 2 should not be particularly joined by welding but rather they may be constructed integrally, if the first and second members 1 and 2 may be made of the same material one another.
It may be also contemplated that a coating or plating treatment may be applied to form a protective film after welding in order to prevent corrosion of the alloy layer 4 and the first metallic member 1 exposed from the tin or silver film 3 from generating.
However, since the size of the welded portion is relatively small and other elements are disposed close in the vicinity of the welded portion when resistance welding is performed for bonding between the terminal member and the connecting conductor in electrical connection thereto in the electronic component, it is often very difficult to properly form the protective film on the welded portion and in the vicinity thereto.